Compensating network



n 1954 J, 1.. GARRISON ET AL 8 COMPENSATING NETWORK Filed Oct. 19. 1950n we mu Rm 1 1.1114 MW 6 LP 2 m m J R w m S W 4 o C G E a 1 5 n I 0 W l.c t R/ N 4 1| 1.. 2E 2 P fig 2 WM 9/ m I 2 1 4 m /z\||1 .0 Rs R 1% 4 o HG B A 0 E wmmwmtw \S kzwoiau Qwwbxmoz @MfiW ATTORNEY Patented June 1,1954 UNITED STATES PATENT OFFICE COMPENSATING NETWORK Jedediah L.Garrison, Madison, N. J., and John P. Whistler, San Marino, Calif.,assignors to Bell Telephone Laboratories, Incorporated, New

York

, N. Y., a corporation of New York Application October 19, 1950, SerialNo. 191,006

Claims.

tional voltage pulses in a circuit which includes a magnetic-coredinductor.

A more specific object is to obtain a rectangular voltage pulse from aunidirectional voltage pulse impressed upon a magnetic-cored coil.

, When a unidirectional voltage pulse is impressed upon a resistive loadwhich has a magnetic-cored inductor connected in shunt or in seriestherewith, neither the voltage across the load nor the currenttherethrough remains constant over the period of the applied pulse. Thisis due to the fact that the magnetizing currentv taken by the inductoris not constant over this period. This current rises rapidly at first,as the eddy current flux is established throughout the core structure,and then gradually becomes substantially linear. Since the current drawnfrom the source varies, its terminal voltage also varies, due to thevoltage drop caused by its interval impedance. Therefore, both thevoltage supplied to, and the current through, the load will varyaccordingly. Such a varying voltage and current are, in many cases,undesirable. For example, a rectangular voltage pulse at the load may berequired, or a constant current to actuate a current-operated device.

In accordance with the present invention a compensating network isassociated with the inductor to make the load current and voltage sub-vstantially constant. In the embodiments shown the network comprisesalplu'rality of impedance branches connected. effectively in parallelwiththe inductor. Eachbran'ch includes a resistance One or.

and a capacitance connected in series. more of the branches are designedto make the current drawn from the voltage source substantially linearover the period of the voltage pulse, and one or more other branches aredesigned to make this current substantially constant over this period.When this current is constant, the ohmic drop in the source is constant,the terminal voltage remains constant, and the load volt age and currentremain constant throughout each pulse period.

The nature of the invention will be more fully understood from thefollowing detailed description and by reference to the accompanyingdrawings, of which:

Fig. *1 is a schematic circuit of one embodiment of a compensatingnetwork in accordance with the invention in which the inductor shuntsthe load;

Fig. 2 shows the compensating network of Fig. 1 applied to atransformer;

Fig. 3 shows current-time characteristics used in a graphicaldetermination of certain factors appearing in the design formulas forthe component elements of the network; and

Fig. 4 is a schematic circuit of another embodiment of the invention inwhich the load and from the source E substantially linear over the theinductor are connected in series.

Taking up the figures in greater detail, Fig. 1 shows a source ofunidirectional rectangular pulses of voltage E and internal impedanceRs,

a resistive load of impedance R1. connected to the source, an inductordesignated by its inductance L connected in shunt with the load, and acompensating network in accordance with the invention comprising threeimpedance branches 40, H and I2 all connected in shunt with the inductorL. If more precise compensation is required, the compensating networkmay include additional shunt branches, as indicated by the broken linesl3. Each of the branches comprises a resistance and a capacitanceconnected in series. In the branch ill the resistance has a value R1 andthe capacitance a value C1, in the branch H these elements are R2 and02, respectively, and in the branch [2 they are Rn and Ca.

As indicated, inductor L is assumed to have a magnetic core associatedtherewith and. may,

for example, be a retardation or choke coil, the

winding of a re1ay,.or the mutual inductance between the primary andsecondary windings of a The latter case is illustrated by Fig; l'exceptthat the'inductor' L "has been replaced by a transformer M with aprimary wind: ing l5, '.a secondary winding l6, and a magnetic core II.The branches Iii, H and i2 may be placed on the primary side of thetransformer 14, as shown, or one or more of them may be transferred tothe secondary side, if the values of their component elements arechanged to take account of the transformation ratio of the transformer.

In its simplest form the compensating network consists of only the twobranches i0 and l I. The circuit will, therefore, be as shown in Fig. 1if the branch 2 is open-circuited or removed. In accordance with theinvention, the values of the resistance R1 and the capacitance C1 in thebranch iii are shown to make the current drawn period T4 of theunidirectional voltage pulse and the value of R2 and C2 in the branch Hare chosen to make this current substantially constant over the periodT4. With this current constant, it follows that the voltage drop acrossthe source impedance Rs will be constant, the terminal voltage will beconstant and, therefore, the voltage across and the current through theload R-L will also be constant. As a result, a rectangular voltage pulsewill be impressed uponthe load RL.

The value of R1 depends upon the magnitudes of the source impedance Rs,the load impedance R1,, and a certain normalized current B. The value ofC1 is dependent upon these same factors and a time constant T1. Thevalue of R2 depends upon Rs, R1,, B, and the normalized steady statevalue H of the magnetizing current, and the value of C2 is dependentupon the same factors and a time constant T3. The factors B, T1 and T3may be determined by the characteristic of the magnetizing current im'drawn by the inductance L from the source E over the pulse period Ti.Explicitly, these element values are given by the following formulas:

1 RSRL R1" RS+RL R R O RS+RL) 2 1. RgR HB RS+RL and R R C RS+RL)' Agraphical method of determining the required values of the factors B, T1and T3 will now be presented with the aid of Fig. 3 in which normalizedcurrent in amperes is plotted against the time t in seconds over theperiod T4 of the pulse. The solid-line curve It gives the normalizedmagnetizing current i111 drawn by the inductor L. The normalized current2111 is obtained by dividing the magnetizing current im', indicated inFig. 1, by the voltage E of the source. The current im may be found withthe aid of a cathode-ray oscilloscope or it may be computed, forexample, from information presented in a paper by A. G. Ganz entitledApplications of Thin Permalloy Tape in Wide Band Telephone and PulseTransformers, published in the Transactions Section of ElectricalEngineering, vol. 65, April, 1946, pages 177 through 183.

It will be noted that the curve it rises sharply at first and then moregradually until at the point d, corresponding to the time T2, it becomessubstantially linear and remains linear for the rest of the pulse periodT4. The required value of the normalized current B is obtained byextending the linear portion of the curve [8 to the left, as shown bythe broken line i9, until it intersects the current axis.

Next, a perpendicular line 213 is dropped from the point on the curve IEto the time axis and a horizontal line 2i is drawn from the current axisat the point B to intersect the perpendicular 26 at the point f. Thenthe broken line curve 22 is constructed by lowering the curve 18 at eachpoint by a. distance equal to the difference between the curves i9. and21. starts at zero and rises smoothly to the point f.

Thus, curve 22 Now, to find the time constant T1, a horizontal line isdrawn from a point A on the current axis equal to 0.632 B until itintersects the curve 22 at the point It, and a perpendicular is droppedfrom this point to the time axis to find T1.

If the values of B and T1 thus found are substituted in Formulas 1 and 2values of R1 and C1 will be found such that, if only the compensatingbranch H] is employed, the current drawn from the source E by thisbranch and the inductor L will be substantially linear over the pulseperiod T4, as shown by the straight line i9 and the straight portion ofthe curve i8. As already pointed out, however, this current should notonly be linear but should also be constant. The compensating branch H isadded for this purpose. The method employed is to build up the linearcurrent already obtained to a constant value equal throughout the pulseperiod to the steady state value H shown by the horizontal line 26 Therequired value of the resistance R2 is found from Equation 3. The timeconstant T3 may be found graphically by drawing a horizontal line 25from a point G on the current axis, where until it intersects the curveIt at the point 1n, and then dropping a perpendicular line 26 from thispoint to meet the time axis at Ta. The required capacitance C2 may nowbe found from Equation 4.

If the compensating network employs only the two branches [0 and H withthe element values determined as explained above, the current drawn fromthe source E will be substantially constantover the pulse period T4, asshown by the curve 24, the ohmic drop in the source impedance He will besubstantially constant, and the voltage across and current through theload R1. will be nearly enough constant for most applications. However,a further improvement in the constancy of this current and voltage maybe made, if required, by adding one or more impedance branches to aidthe branch [0 in making the current linear, and one or more branches toaid the branch I I in making the current constant. Each of theseadditional branches preferably comprises the series combination of aresistance and a capacitance, and the branches are connected in shuntwith the inductance L. The branch I2 shown in Fig. 1 is one such addedbranch. If additional branches are used, the required values of theelements R1, C1, B2 and C2 will, in general, differ from those obtainedby the formulas presented herein. The required values of the componentresistances and capacitances in the added branches may be found fromadditional normalized currents and time constants which may be foundfrom the characteristic of the magnetizing current shown in Fig. 3 by agraphical method similar to the one described above.

When the compensating network is used with a transformer, as in thecircuit of Fig. 2, the

' required values of the component elements in the branches l0 and I!may be found in the manner already described, except that in this casethe magnetizing current characteristic [8 will, of course, apply to thetransformer I4 instead of the inductance L.

Another embodiment of the invention is shown in Fig. 4, which is thesame circuit as that shown. in Fig. 1 except that. the shunt load R1. isomitted and a series load of impedance R2 is connected between thesource E and. the first shunt branch [0. In this case the requiredvalues of the elements in the branches l0 and I l to make the currentdrawn from the source E substantially constant throughout the period T4of the voltage pulse are given by the following formulas:

and

in which the factors B, H, T1 and T3 are determined in the mannerpreviously described.

In the circuit of Fig. 4 if only the two compensating branches l0 and Hare used the current through and voltage across the load RP will besubstantially constant over the pulse period T4. The constancy may befurther improved, if required, by the addition of one or more branchessuch as 12, as explained above in connection with Fig. 1.

What is claimed is:

1. In combination, a resistive load of impedance Z adapted forconnection to a source of unidirectional voltage pulses of internalimpedance Rs, a magnetic-cored inductor connected in shunt with saidload, and a compensating network for making the voltage across and thecurrent through said load substantially constant over the pulse period,said network comprising a plurality of impedance branches connectedeffectively in shunt with said inductor, each of said branchescomprising the series combination of a resistance and a capacitance, theresistance R1 and capacitance C1 in one of said branches having valueschosen to make the current drawn from said source substantially linearover said period, and the resistance R2 and capacitance C2 in a secondof said branches having values chosen to make said last-mentionedcurrent substantially constant over said period, in which R1, 01, R2 andC2 have approximately the following values:

ing the first curve at each point by a distance equal to the diflerencebetween the extended porto 0.632 B, and reading the time T3 at whichsaid first curve attains a value equal to 2. In combination, a resistiveload of impedance Z adapted for connection to a source of unidirectionalvoltage pulses of internal impedance Rs, a magnetic-cored inductorconnected in series with said load, and a compensating network formaking the voltage across and the current through said loadsubstantially constant over the pulse period, said network comprising aplurality of impedance branches connected effectively in shunt with saidinductor, each of said branches comprising the series combination of aresistance and a capacitance, the resistance R1 and capacitance C1 inone of said branches having values chosen to make the current drawn fromsaid source substantially linear over said period, and the resistance R2and capacitance C2 in a second of said branches having values chosen tomake said last-mentioned current substantially constant over saidperiod, in which R1, C1, R2 and C: have approximately the followingvalues:

where H is the normalized value of the steady state magnetizing currentdrawn from said source by said inductor, and the factors B, T1 and T2may be found graphically by plotting a first curve of the normalizedmagnetizing current in amperes against time over one period of saidpulses, extending linearly the linear portion of the curve to thecurrent axis to find the point of interception B, constructing a secondcurve by lowering the first curve at each point by a distance equal tothe difference between the extended portion of the first curve and B,reading the time T1 at which the second curve attains a value equal to0.632 B, and reading the time T3 at which said first curve attains avalue equal to 0.632(H-B) +B.

3. In combination, a resistive load of impedance Z adapted forconnection to a source of unidirectional voltage pulses of internalimpedance Rs, a magnetic-cored inductor connected in parallel with saidload, the series combination of a resistance R1 and a capacitance C1 ina path shunting said inductor for making the current drawn from saidsource substantially linear over the pulse period, and the seriescombination of a resistance R2 and a capacitance C2 in a second pathshunting said inductor for making said current substantially constantover said pulse period, in which R1, C1, R2 and C2 have approximatelythe following values:

R 1, m..li? "'H-B R -i-Z and R Z s-lZ R2 where H is the normalized valueof the steady state magnetizing current drawn from said source by saidinductor, and the factors B, T1 and T2 may be found graphically byplotting a first curve of the normalized magnetizing current in amperesagainst time over one period of said pulses, extending linearly thelinear portion of the curve to the current axis to find the point ofinterceptime B, constructing a second curve by lowering the first curveat each point by a distance equal to the difference between the extendedportion of the first curve and B, reading the time T1 at which thesecond curve attains a value equal to 0.632 B, and reading the time T3at which said first curve attains a value equal to 0.632(HB) +B 4. Thecombination in accordance with claim 3 which includes an additionalimpedance branch shunting said inductor, said additional branchcomprising the series combination of a resistance and a capacitanceproportioned to make said Z adapted for connection to a source ofunidirectional voltage pulses of internal impedance Rs, a magnetic-coredinductor connected in series with said lead, the series combination of aresistance Rd and a capacitance C1 in a path shunting said inductor formaking the current drawn from said source substantially linear over thepulse period, and the series combination of a resistance R2 and acapacitance C2 in a second path shunting said inductor for making saidcurrent substantially constant over said pulse period, in which R1, 01,R2 and C; have approximately the following values:

where H is the normalized value of the steady state magnetizing currentdrawn from said source by said inductor, and the factors B, T1 and T2may be found graphically by plotting a first curve of the normalizedmagnetizing current in amperes against time over one period of saidpulses, extending linearly the linear portion of the curve to thecurrent axis to find the point of interception B, constructing a secondcurve by lowering the first curve at each point by a distance equal tothe difference between the extended portion of the first curve and B,reading the time T1 at which the second curve attains a value equal to0.632 B, and reading the time T3 at which said first curve attains avalue equal to 8. The combination in accordance with claim 7 whichincludes an additional impedance branch shunting said inductor, saidadditional branch comprising the series combination of a resistance anda capacitance proportioned to make said current more nearly constant.

9. The combination in accordance with claim 7 which includes anadditional impedance branch shunting said inductor, said additionalbranch comprising the series combination of a resistance and acapacitance proportioned to make said current more nearly linear.

10. The combination in accordance with claim 9 which includes a secondadditional impedance branch shunting said inductor, said secondadditional branch comprising the series combination of a resistance anda capacitance proportioned to make said current more nearly constant.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 1,801,342 Gannett et a1. Apr. 21, 1931 2,035,457 Blumlein Mar,31, 1936 2,317,482 Peterson Apr. 27, 1943 2,431,952 Maxwell Dec. 2, 19472,470,825 Mathes May 24, 1949 2,480,511 Schade Aug. 30, 1949

